1. Field of the Invention
Aspects of the present invention relate to an Si/C composite, anode active materials, and a lithium battery including the same. More particularly, aspects of the present invention relates to an Si/C composite having a high charge/discharge capacity and a good capacity retention, anode active materials, and a lithium battery including the same.
2. Description of the Related Art
Various conventional techniques for using of lithium metals as anode materials for lithium batteries have been suggested. However, when lithium metals are used as anode materials, short circuits of a battery may occur due to lithium dendrite formation, which creates a high risk of explosion. Thus, to overcome such shortcoming, carbonaceous active materials have been widely used as anode materials instead of lithium metals. Examples of carbonaceous active materials include crystalline carbon, such as natural graphite and artificial graphite, and amorphous carbon, such as soft carbon and hard carbon. However, although amorphous carbon has good capacity, when used, many of the charge/discharge reactions are irreversible. Since the theoretical capacity of crystalline carbon (such as, for example, graphite) is relatively high, i.e., 372 mAh/g, crystalline carbon has been widely used as an anode active material. However, although the theoretical capacity of such graphite- or carbon-based active materials is currently considered to be rather high, the theoretical capacity is not high enough for future lithium batteries, which may require higher capacities.
To address these problems, research into metal-based anode active materials and intermetallic compound-based anode active materials has been actively conducted. For example, research into lithium batteries using metals or semimetals such as aluminum, germanium, silicon, tin, zinc, lead, etc., as the anode active materials has been conducted. Such materials are known to have large capacities, high energy densities, and good insertion/extraction capabilities compared to carbon-based anode active materials. Thus, lithium batteries having large capacities and high energy densities can be prepared using these materials. For example, pure silicon is known to have a high theoretical capacity of 4017 mAh/g.
However, such materials typically have shorter life cycles than carbon-based materials, and thus are difficult to put to practical use. When inorganic particles such as silicon or tin are used as the anode active material, the volume of the inorganic particles changes considerably during charge/discharge cycles. This may result in the degradation of the electronic conduction network between the active material particles or may result in the detachment of the anode active material from the anode current collector. That is, the volume of inorganic material such as silicon or tin increases by about 300 to 400% due to alloying with lithium during charging, and the volume decreases due to extraction of lithium during discharging. Therefore, after repeated charge/discharge cycles, spaces may be generated between the active material particles, and electrical insulation may occur, thereby rapidly lowering the capacity retention of these materials, thereby causing a serious problem when these materials are used in lithium batteries.
To overcome these disadvantages, research into methods of absorbing the volume expansion of metal particles by using nano-sized silicon particles or porous silicon has been carried out. With regard to the nano-sized silicon particles, Japanese Laid-Open Patent Application Publication No. 1998-003920 discloses a lithium secondary battery having metallic nanoparticles coated with carbon. However, the nanoparticles are quite expensive to produce, the carbon (which is brittle) on the surface of the nanoparticles cracks due to expansion during charging, and spaces are generated between carbon and metal nanoparticles due to contraction during discharging. Therefore, improvements in battery life cycle are restricted. With regard to porous silicon, several methods have been disclosed. For example, Japanese Laid-Open Patent Application Publication No. 2004-327330 discloses the use of cathodic oxidation. Korean Patent Publication No. 2004-0063802 discloses a preparation method of a negative active material in which an alloy is formed of Si and an elemental metal such as Ni, and the elemental metal is then eluted. Also, Korean Patent Publication No. 2004-0082876 discloses a preparation method of porous silicon comprises mixing an alkali metal or alkali earth metal powder with a silicon precursor powder such as silicon dioxide, and heating the mixture under inert atmosphere, followed by eluting with acid. Although these methods demonstrate an improvement in the initial capacity retention to some extent owing to the absorbing effects of the volume expansion derived from porous structures, the use of only porous silicon particles with low conductivity may lower the electric conductivity among particles in the course of manufacturing an electrode plates unless nanoparticles are used, ultimately deteriorating the initial coulombic efficiency or the capacity retention. However, according to these methods, it is not possible to prepare nanoparticles, or additional processing costs may be involved.
Accordingly, there is a need for an anode active material with high initial coulombic efficiency and good capacity retention using a material that can improve the conductivity of particles.